El objetivo del presente estudio es conocer la variación estacional de las especies del fitoplancton y su relación con las condiciones ambientales durante el período en estudio.It is presented the investigations of clorofila a cell counts, and qualitative (net 50u), that used simultaneously provided valuable ecological information on the community of the phytoplankton accomplished in the interior Gulf of Guayaquil, between 11 august of 1999, 14 of march of 2000 and 14rd of may/2001. They were identified 81 species in water samples and 80 species in net samples (50u), in the community of the phytoplankton. The diatoms prevailed dominant and in smaller concentration the dinoflagellates, coccolithophores, cianobacters, flagellates and tintinnoids. In cell counts Skeletonema costatum, Microspora tumidula y Mesodinium rubrum they were the species of greater ocurrence in all the area. In net samples (50u), were registered species typical of environment estuary: Skeletonema costatum, Ulotrix aequalis, Cyclotella menenghiniana, Chaetoceros vistulae, Pleurosigma angulatum, Actynoptichus senaritus. The distribution and dominance of species presented a which one characteristic for the area. Furthermore, they were observed some species of character benthic.